Channel division

ABSTRACT

A communication device that forms part of a network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the communication device being configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in its neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with of those communication devices on a time division basis.

The present invention relates to assigning frequency hopping sequences to cells in a cellular communication network.

A wireless network may be configured to operate without having been specifically allocated any part of the electromagnetic spectrum. Such a network may be permitted to operate in so-called whitespace: a part of the spectrum that is made available for unlicensed or opportunistic access. Typically whitespace is found in the UHF TV band and spans 450 MHz to 800 MHz, depending on the country. A large amount of spectrum has been made available for unlicensed wireless systems in this frequency range.

A problem with operating in whitespace is that the available bandwidth is variable and cannot be guaranteed. These limitations are well-matched to the capabilities of machine-to-machine networks in which there is no human interaction. Machine-to-machine networks are typically tolerant of delays, dropped connections and high latency communications.

Any network operating in the UHF TV band has to be able to coexist with analogue and digital television broadcast transmitters. The density of the active television channels in any given location is relatively low (resulting in the availability of whitespace that can be used by unlicensed systems). The FCC has mandated that systems operating in the whitespace must reference a database that determines which channels may be used in any given location. This is intended to avoid interference with the TV transmissions and certain other incumbent systems such as wireless microphones.

For TV receivers (including those for digital TV (DTV)), there will inevitably be adjacent channels on which a strong transmission close to the TV receiver will interfere with TV reception. For example, the TV receivers may have image frequencies and poor adjacent channel rejection (ACR) on certain frequencies due to spurs on their local oscillators and limitations in their receive filters. These frequencies are often dependent on the specific receiver implementation.

Digital TV typically uses a channel bandwidth of 6 to 8 MHz. It also uses OFDM modulation in which the overall channel bandwidth is split into a large number of narrower channels (so-called sub-carriers), each of which is individually modulated. The system is designed so that, if a certain number of sub-carriers are subject to multipath fading, with the result that their signal-to-noise ratio is poor, the overall data can still be recovered. This is typically achieved by using interleaving and error correction codes, which mean that bit errors localised to a limited number of sub-carriers can be corrected. OFDM modulation can therefore achieve considerable robustness to multipath fading.

OFDM is only able to recover the transmitted data when the interferer is relatively narrowband compared with the bandwidth of the overall TV signal, such that a limited number of sub-carriers are affected. OFDM does not provide a similar performance benefit when the interferer occupies a relatively large proportion of the DTV channel bandwidth because in this case the error control coding may be incapable of correcting the bit errors due to the higher proportion of bits that may be corrupt. If the bandwidth of the transmitted signal from the terminal can be reduced to a small fraction of the DTV channel bandwidth, there is a lower chance of the DTV receiver being unable to decode the signal correctly. Another perspective on this is that the narrowband whitespace transmitter can be located much closer to the DTV receiver before causing noticeable degradation of the decoded DTV signal. This can be of particular benefit for mobile or portable whitespace devices whose exact location and antenna orientation cannot be easily constrained.

There is a potential issue with reducing the bandwidth occupied by the whitespace device's transmitter: transmitting on a narrow bandwidth channel makes the whitespace device sensitive to poor reception due to multipath fading. This is because the entire bandwidth could be in a long-term fade (lasting multiple frames), resulting in poor signal-to-noise ratio.

Both of these problems may be addressed using frequency hopping. Frequency hopping minimises the interference to TV reception, since no communication will be permanently causing interference to any given TV receiver. Frequency hopping also reduces the probability of the terminal being in a long-term fade. It provides a form of interleaving that enables more efficient error correction to be used.

The channels used for frequency hopping may be selected by the base station based upon information from the whitespace database on the available channels and associated power levels (which in turn are based upon the licensed spectrum use in the area). However, the whitespace database does not include information about every possible source of interference.

For example, a television transmitter may be intended to broadcast to only a particular coverage area, but may in fact leak into other nearby areas where the use of the frequencies in use by that transmitter are not prohibited in the whitespace database; major TV stations can be well above the thermal noise at distances of 100 km. Although the signal from this transmitter may not be strong enough to be reliably received by television antennas in those nearby areas, it is often strong enough to cause severe interference to whitespace base stations in those areas, particularly if they have elevated antennas (which they may have in order to increase their own coverage area). On nominally free channels, reception is far more likely to be dominated by distant TV broadcasts rather than thermal noise, especially in rural regions. This interference can render many of the whitespace channels unusable or severely compromised.

Interference from other unlicensed whitespace networks can also be a problem as all whitespace networks compete for use of those frequencies the whitespace database marks as available.

Interference may also be caused by the unintended emissions of devices that are not part of a wireless network, e.g. spurious emissions from faulty electric drills.

Apart from all these interferers external to the network, there can also be problems for devices located close to the edge of cells. Neighbouring base stations are likely to have similar whitespace channel assignments. (As the distance between base stations increases, the assignments tend to change as the base stations are located in different TV service areas.) Therefore, if base stations pick their own frequency hopping sequences based on only the frequencies available in the whitespace database, the base stations of neighbouring cells are likely to make similar choices. If neighbouring cells use the same frequency hopping sequences then terminals at cell edges may receive multiple weak signals from both the base station for their own cell, and any neighbouring base stations in range, and have no way of distinguishing between them. Two neighbouring base stations may use the same frequencies on approximately one in ten frames. Each base station is surrounded by a number of others, typically around six, meaning interference is likely to occur somewhere within each cell around fifty percent of the time. This can result in a significant loss in capacity.

What is needed is a method and apparatus for optimising the frequencies that are available to cellular communication networks such as those operating in whitespace.

According to a first embodiment of the invention, there is provided a communication device that forms part of a network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the communication device being configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in its neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with those communication devices on a time division basis.

The communication device may be configured to share the insufficient number of frequencies with its neighbouring communication devices responsive to a communication from a network controller.

The communication device may be configured to share the frequencies responsive to a command from the network controller.

The communication device may be configured to determine that it should share the frequencies responsive to a frequency hopping sequence received from the network controller.

The communication device may be configured to determine that it should share the frequencies responsive to the number of frequencies comprised in the frequency hopping sequence being below a predetermined threshold.

The communication device may be configured to negotiate with its neighbouring communication devices to determine how the insufficient number of frequencies should be shared between them.

The communication device may be configured to communicate with the at least one terminal comprised in its cell via a series of frames, each frame comprising a downlink portion and an uplink portion, and share the insufficient number of frequencies with its neighbouring communication devices by communicating the downlink portion of a frame in one timeslot on a frequency in its frequency hopping sequence and the uplink portion of the same frame in a different timeslot on the same frequency.

The communication device may be configured to communicate the downlink portion and the uplink portion of the frame in timeslots arranged such that the time from the start of the timeslot for the downlink portion to the end of the timeslot for the uplink portion is less than or equal to the duration of a frame via which the communication device normally communicates with the at least one terminal.

The communication device may be configured to communicate the downlink portion and the uplink portion of the frame in timeslots that are separated by one or more timeslots allocated to its neighbouring communication device(s).

The communication device may be configured to communicate the downlink and uplink portions of the frame such that the at least one terminal perceives the one or more timeslots allocated to the neighbouring communication device(s) as being parts of the frame that are allocated to other terminals in the cell.

According to a second embodiment of the invention, there is provided a communication network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell and each communication device being further configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with those communication devices on a time division basis.

The communication network may be configured to determine that there are an insufficient number of frequencies available for a group of two or more neighbouring communication devices if the frequency hopping sequences derivable from those available frequencies require two or more neighbouring communication devices to use the same frequency for a length of time that is likely to lead to unacceptable packet loss.

The communication network may comprise a controller configured to determine that there are an insufficient number of frequencies available to provide frequency hopping sequences for a group of two or more neighbouring communication devices. The controller may be configured to communicate that there are an insufficient number of frequencies to the two or more communication devices. The controller may be configured to instruct the two or more communication devices to share the insufficient number of frequencies on a time division basis.

The communication network may be configured to allocate neighbouring communication devices the same length of time on each of the insufficient number of frequencies.

The communication network may be configured to allocate neighbouring communication devices different lengths of time on each of the insufficient number of frequencies.

The communication network may be configured such that each of the communication devices communicates with its at least one terminal via frames having the same duration, those communication devices being configured to, when sharing a frequency with a neighbouring communication device, commence transmitting each frame at a different time from the neighbouring communication device so that the frames of one communication device are offset from those of another.

The communication network may be configured to operate in whitespace.

The communication network may be configured for machine-to-machine communication.

According to a third embodiment of the invention, there is provided a method for communicating in a network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the method comprising, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for two or more communication devices in neighbouring cells that are sufficiently distinct from each other, sharing the insufficient number of frequencies between those communication devices on a time division basis.

Aspects of the present invention will now be described by way of example with reference to the accompanying drawings. In the drawings:

FIG. 1 shows an example of a machine-to-machine network;

FIG. 2 shows an example of a frame structure;

FIG. 3 shows an example of a process that may be implemented by a controller;

FIGS. 4( a) to 4(c) show examples of frequency hopping sequences;

FIG. 5 shows an example of time sharing between four neighbouring communication devices;

FIG. 6 shows an example of a process for sharing frequencies on a time division basis;

FIG. 7 shows an example of a controller; and

FIG. 8 shows an example of a communication device.

A background aspect of the invention relates to gathering information about frequency availability in each cell, and in dependence on that information assigning frequency hopping sequences to the cells in such a way as to reduce message failure caused by interference.

The communication network may be a wireless network comprising a number of cells. Preferably each cell contains a communication device, such as a base station, and one or more communication terminals with which the communication device is capable of communicating according to a frequency hopping communication sequence. The central controller gathers information relating to the frequency availability in each cell and uses this to determine appropriate frequency hopping sequences for each cell. These frequency hopping sequences are then communicated to the communication devices. The communication devices then suitably communicate the frequency hopping sequence appropriate to their respective cells to the terminals in the cell and subsequently use that hopping sequence in communicating with the terminals.

It is preferable for the frequency hopping sequences to be determined in such a way as to minimise (and preferably eliminate) periods of time during which communication devices in neighbouring cells are using the same frequency. Sometimes, however, there may simply not be enough frequencies available for this to be possible. If it is determined that for a particular group of two or more neighbouring communication devices there are an insufficient number of frequencies available to provide adequate frequency hopping sequences for all of them, the communication devices suitably share the frequencies that are available on a time division basis.

One or more embodiments of the invention will now be described with specific reference to a wireless network in which the communication devices are base stations. This is for the purposes of example only and it should be understood that the frequency hopping sequence allocation mechanism described herein may be implemented by any suitable communication devices, irrespective of what particular role those devices play within the network. Also, one or more embodiments are described with specific reference to wireless networks in which the controller is an operations centre for the network. It should be understood that this is also for the purposes of example only and the frequency hopping sequence allocation mechanism described herein may be generated and implemented by any suitable device (including, for example, a base station).

An example of a wireless network is shown in FIG. 1. The network, shown generally at 104, comprises one or more base stations 105 that are each capable of communicating wirelessly with a number of terminals 106. Each base station may be arranged to communicate with terminals that are located within a particular geographical area or cell. The base stations transmit to and receive radio signals from the terminals. The terminals are suitably entities embedded or machines or similar that communicate with the base stations. Suitably the wireless network is arranged to operate in a master-slave mode where the base station is the master and the terminals are the slaves.

The base station controller 107 is a device that provides a single point of communication to the base stations and then distributes the information received to other network elements as required. That is, the network is based around a many-to-one communication model. The network may be arranged to communicate with a client-facing portion 101 via the internet 102. In this way a client may provide services to the terminals via the wireless network.

Other logical network elements shown in this example are:

-   -   Core network. This routes traffic information between base         stations and client networks.     -   Billing system. This records utilisation levels and generates         appropriate billing data.     -   Authentication system. This holds terminal and base station         authentication information.     -   Location register. This retains the last known location of the         terminals.     -   Broadcast register. This retains information on group membership         and can be used to store and process acknowledgements to         broadcast messages.     -   Operations and maintenance centre (OMC). This monitors the         function of the network and raises alarms when errors are         detected. It also manages frequency and code planning, load         balancing and other operational aspects of the network.     -   Whitespace database. This provides information on the available         whitespace spectrum.     -   Client information portal. This allows clients to determine data         such as the status of associated terminals, levels of traffic         etc.

In practice, many of the logical network elements may be implemented as databases running software and can be provided on a wide range of platforms. A number of network elements may be physically located within the same platform.

A network such as that shown in FIG. 1 may be used for machine-to-machine communications, i.e. communications that do not involve human interaction. Machine-to-machine communications are well-matched to the limitations of operating in whitespace, in which the bandwidth available to the network may vary from one location to another and also from one time instant to the next. As the network does not have any specific part of the spectrum allocated to it, even unallocated parts of the spectrum may become unavailable, e.g. due to a device in the vicinity that is operating outside of the network but using the same part of the spectrum. Machines are able to tolerate the delays and breaks in communication that can result from these varying communication conditions. Services can be provided in non real-time; low latency is not important as long as data is reliably delivered.

The network may use medium access control (MAC) to share the same radio resource between multiple terminals. An example of a suitable frame structure is shown in FIG. 2. The frame (shown generally at 201) comprises time to ramp-up to full output power 202 (T_IFS), a synchronisation burst 203 (DL_SYNC), an information field providing the subsequent channel structure 204 (DL_FCH), a map of which information is intended for which terminal 205 (DL_MAP), a field to allow acknowledgement of previous uplink transmissions 206 (DL_ACK) and then the actual information to be sent to terminals 207 (DL_ALLOC). There is then a guard period for ramp-down of the downlink and ramp-up on the uplink 208 (T_SW), followed by the allocated uplink data transmissions 210 (UL_ALLOC) in parallel with channels set aside for uplink contended access 209 (UL_CA).

A suitable hopping rate for the downlink channels may be the frame rate, so that each frame is transmitted on a different frequency from the preceding frame. The frames for a network designed to operate in whitespace for machine-to-machine communication may be particularly long. In one example the frames may each be 2 seconds long, giving a frequency hop on the downlink every 2 seconds (which is 30 hops per minute).

The DL_FCH may include information to enable the terminals to determine the hopping sequence. The DL_FCH may include a list of the frequencies that are included in the sequence. One efficient way of communicating this information is by means of a channel map, with a bit being set if the channel is in use in the base station. The DL_FCH may also include a MAC Frame count (16-bit) enabling terminals to determine where the base station is in its hopping pattern.

The DL_MAP informs terminals as to whether there is any information for them in the frame and whether they have an uplink slot reserved for them to transmit information.

It comprises a table of terminal identities, the number of slots that their information is spread over and the transmission mode and spreading factors used. All terminals monitoring the frame decode this field to determine whether they need to decode subsequent information. The length of the DL_MAP may be included as part of the DL_FCH. A terminal can determine the position of its assigned slots from the DL_MAP by adding up the number of slots allocated in prior rows in the table.

On the uplink the slots may be numbered from 0 to n on the first FDMA channel, then on the subsequent FDMA channel and so on. The terminal can determine how many slots there are each channel from the length of the frame available for the uplink (that remaining after completion of the downlink) divided by the length of each slot. If a terminal has data requiring multiple slots it would normally be given these consecutively on the same carrier as this both simplifies the terminal transmission and minimises the control information required to describe the slot location. However, it is possible to give the terminal multiple allocations on different carriers (so long as they are not simultaneous) to achieve frequency hopping on the uplink.

This combination of DL_SYNC, DL_FCH and DL_MAP is collectively known as the “header”. In an M2M system, with lower data rates achieved through spreading, the header will typically last between 20 ms and 200 ms of the 2 second frame (depending on the number of allocations to be signalled in the DL-MAP field).

In order to increase the number of messages that are reliably delivered, a central controller may be provided to make intelligent frequency hopping sequence allocations to cells using analysis of frequency availability information. A suitable process that may be performed by the controller is shown in FIG. 3. The process commences in step 301. In step 302, the controller determines, for each and every cell, which frequencies are permitted for whitespace use. The controller may perform this step by accessing the whitespace database to rule out those frequencies reserved for licensed users. The controller may then determine what frequencies are otherwise excluded as being unsuitable (step 303). It may, for example, rule out as being unsuitable frequencies on which an unacceptably high level of interference has been found. In step 304 the controller produces a finalised list of frequencies that are available to each cell. The controller uses this list to generate a frequency hopping sequence for each cell (step 305), which is then communicated to the appropriate base station (step 306). The process finishes in step 307.

Once the controller has established which frequencies are available for use in each cell it can start to allocate frequency hopping sequences. It is preferable for the sequences to contain as many frequencies as possible to reduce the impact of fading etc, as discussed above. However, the sequences should also be generated so as to minimise the occasions on which neighbouring cells will be transmitting on the same frequency, as this can cause interference to the terminals in each cell (particularly those located near to a cell boundary). The controller may employ an algorithm to determine every possible frequency sequence across the cells of the network to analyse which arrangement will generate the least amount of overlap between neighbouring cells.

If the number of available frequencies is high, and the network relatively large, a computation such as the one described above can rapidly require an unworkable level of resource. A preferred option is for the available frequencies to be arranged in a predetermined order, with each cell starting its respective hopping sequence at a different frequency in the order from its neighbouring cells. The predetermined order might be random or worked out according to some rule. For example, the available frequencies might simply be organised into ascending or descending order. An example is shown in FIG. 4( a). Cells 1 to 3 are neighbouring cells and frequencies 1 to 4 are available in each cell. Each cell is assigned a cyclic-sequence in which frequencies 1 to 4 are used in ascending order. However, each cell commences its respective sequence at a different offset from its neighbour, so that at any given time each cell is using a different frequency in the sequence from its neighbour. Simulations have shown that such offset, cyclic frequency hopping sequences functions very well, without the unfeasible computational burden associated with looking at all possible frequency hopping sequences across all cells.

Generating the sequences to simply comprise a list of available frequencies arranged in a predetermined order and then applying a respective offset for each cell works particularly well in networks arranged to operate in whitespace. This is because the frequencies available for use in whitespace are largely dictated by the frequencies that are already allocated to TV channels. Different TV transmitters may use different frequencies (which is why the spectrum available to whitespace networks is dependent on the location of that network); however, each TV transmitter is associated with a large geographical region. Typically, a transmitter may cover an area having a radius of around 50 miles. This means that neighbouring cells will largely have the same set of frequencies available to them. When this is the case, neighbouring cells can be prevented from overlapping in their frequency hopping sequences simply by applying an offset in each cell.

It will not always be the case that neighbouring cells have the same available frequencies, e.g. where neighbouring cells are located in different TV coverage areas. Often, one cell may have more frequencies available to it than its neighbour, which raises a potential problem that the two cells will eventually cycle round to be identical with each other, despite having commenced their cycles with different offsets. An example of such a situation is shown in FIG. 4( b), in which frequencies 1 to 3 are available to cells 1 and 2, but frequency 4 is available only to cell 1. Consequently cell 1 cycles through frequencies 1 to 4, while cell 2 cycles through frequencies 1 to 3 with an offset. However, in this instance, the additional frequency in cell 1's hopping sequence means that the two cells inevitably end up using the same frequency for a time period illustrated by the dotted line 401. The overlap still occurs for a much lower percentage of the time than would be expected if the base stations chose their own frequency hopping sequences, but any overlap will nonetheless cause messages to clash. A solution to this problem would be for cell 1 to remove frequency 4 from its hopping sequence. However, this is not an ideal solution as it is preferred for the hopping sequence of each cell to make use of all the frequencies available to it.

A unique feature of machine-to-machine networks is their ability to tolerate delays caused by message clashes. In a machine-to-machine network, it is usually possible to simply resend data at a later time, with no ill effects, if the first transmission fails. Therefore, for those cells where they will, for some small percentage of the time, be operating on the same frequencies as one or more neighbouring cells, this is not necessarily as problematic as it may first appear. In addition, spreading codes may be implemented to minimise the number of packets lost as a consequence of frequency clashes.

A preferred implementation, however, is to have the cell with fewer frequencies available to it cycle through those frequencies until its neighbouring cell, with more frequencies at its disposal, has finished one of its cycles. This scenario is illustrated in FIG. 4( c). In this example, frequencies 1 to 3 are available to cells 1 and 2, but frequency 4 is available only to cell 1. In this example, the frequency hopping sequence of cell 2 has been extended so that the duration of one cycle matches the duration of one cycle of the frequency hopping sequence of cell 1. This is achieved by extending the set of frequencies through which cell 2 cycles to comprise the same number of frequencies as cell 1. Since four frequencies are not available in cell 2, this requires the first frequency of cell 2's set to be repeated. Another way to view this is that the frequency hopping sequence of cell 2 essentially comprises two different cycles, each with its own periodicity. In the first cycle, cell 2 cycles through frequencies 1 to 3 with its assigned offset. However, that cycle has a duration that is limited to the duration of the cycle of its neighbouring cell, cell 1. In other words, when cell 1 finishes its cycle, the cycle of cell 2 is essentially terminated and reset, so that it recommences its cycle from the beginning. An arrangement such as this may be advantageous in avoiding the overlapping frequencies of the arrangement shown in FIG. 4( b).

In some circumstances there may be insufficient frequencies available to enable neighbouring base stations to be allocated different frequencies frequently enough. For a typical group of base stations, anything fewer than four frequencies may be insufficient. This is because each cell will typically have four neighbours, so that anything fewer than four frequencies makes it impossible for each neighbouring cell to use a different frequency at any given time instant. This may be addressed by having neighbouring base stations share the available frequencies, suitably by using time division multiple access (TDMA).

By sharing the available frequencies between them, each base station may only use a fraction of a frequency. An example is shown in FIG. 5 in which a single available channel (501) is split into eight parts and shared between four base stations (508 to 511) located in neighbouring cells (503 to 507). Base station 509 might be assigned part 1 of the channel for its downlink and part 5 for its uplink. Base station 510 might be assigned parts 2 for its downlink and part 6 for its uplink, and so on. With this approach, the start of the frames is not synchronised across the base stations.

FIG. 5 shows an example in which a single frequency (representing a single available channel) is shared between neighbouring base stations. It may often be the case that more than one frequency is available but still the number of available frequencies is too low for each base station in a group of neighbouring base stations to share them on a purely frequency division basis. This is because the frequency hopping sequences that can be generated using those available frequencies are insufficiently distinct or different from each other, so that implementing them would lead to unacceptably long periods of overlap. Some period of overlap may be acceptable, particularly since machine-to-machine networks are designed to cope well with packet loss. However, periods of overlap lasting longer than a certain threshold are likely to lead to unacceptable levels of packet loss.

In some circumstances, the number of available frequencies available to a group of neighbouring base stations might be compared with the number of base stations in the group and deemed to be insufficient based on some predetermined formula or threshold. In other circumstances, this determination may be made on the basis that the number of frequencies available is too low for each base station in the group to communicate simultaneously via the same frequency. Another option is for the number of available frequencies to be determined to be insufficient in practice, i.e. if it is found that a partial overlap between neighbouring base stations' individual frequency hopping sequences is leading to unacceptable packet loss.

The number of available frequencies may be deemed insufficient by the controller responsible for determining the frequency hopping sequences for the network. In general, it is a central planning system of this kind that will instigate frequency sharing by determining that the frequencies need to be shared and instructing the base stations accordingly. The controller may make this determination based not only on the number of frequencies available, but also information returned by one or more base stations about packet loss, interference from neighbouring base stations etc. Alternatively the number of available frequencies may be deemed insufficient by a base station upon receiving its frequency hopping sequence from the controller, e.g. based on the number of frequencies comprised in that sequence being lower than a predetermined threshold, or on packet loss experienced in the cell when using the frequency hopping sequence. Individual base stations may implement TDMA sharing automatically or after local negotiation with other base stations.

Where more than one frequency is available to each base station, a central controller preferably generates a frequency hopping sequence for each base station by means of the principles and mechanisms described above. Neighbouring base stations may then share a group of available frequencies on both a frequency-division and a time-division basis.

FIG. 5 shows an example in which a single channel is divided up into timeslots of equal length for allocation to the different base stations. Although generally the timeslots would be of equal length, this is not necessary, and different length timeslots may be allocated to different base stations, e.g. if one base station is particularly heavily loaded compared with its neighbours it may be allocated longer timeslots.

FIG. 5 also shows each base station being allocated timeslots for their downlink and uplink communications that are separated by timeslots allocated to other base stations. The base stations could be allocated contiguous timeslots, with no such gap between the uplink and downlink sections. However, an advantage of allocating timeslots that are separated in time is that it enables the uplink and downlink portions of frames from different base stations to be interleaved so that each base station can maintain its normal frame rate. Preferably the time from the start of the timeslot allocated to the downlink to the end of the timeslot allocated to the uplink is less than or equal to the normal frame duration (e.g. 2 seconds). Preferably each downlink timeslot is at least long enough to accommodate the frame header (so at least 200 ms long). Each base station preferably just does not allocate any communications for the part of its frame when the channel is allocated to one of its neighbours. To the terminals, the frame should appear as a normal since they will perceive the ‘gap’ as simply being a part of the frame that is allocated to other terminals in their cell.

Cells and their associated base stations may be considered ‘neighbours’ if they share a boundary. Cells and their associated base stations may also be considered ‘neighbours’ simply if transmissions in one of those cells by the base station or its associated terminals has the capacity to interfere with communications in the other cell.

FIG. 6 shows an example of a process for sharing a limited number of frequencies between a group of base stations. In this example a group of four base stations are sharing two available frequencies: A and B. The process commences in step 601. In step 602 it is determined that frequencies A and B are available to be used by base stations 1 to 4. This number of frequencies is determined to be insufficient (step 603) and the base stations are instructed to share them on a time division basis (step 604). In step 605, base stations 1 and 3 commence their communications on frequencies A and B respectively. The frames of base stations 2 and 4 are offset from those of base stations 1 and 3, and so they commence communications on frequencies A and B in step 606. Steps 605 and 606 are repeated until different frequency hopping sequences are assigned to the base station. The process finishes in step 607.

An example of the functional blocks that may be comprised in a controller according to one embodiment of the invention are shown in FIG. 7. The controller may be comprised in the core network, e.g. the OMC used for operational aspects of the network in a central planning role, or it may be comprised elsewhere in the network, e.g. in one of the base stations. The controller, shown generally at 701, comprises a communication unit 703 connected to an antenna 702 for transmitting and receiving messages. The controller might equally communicate the frequency hopping sequences to the communication devices via a wired connection. The controller further comprises an availability unit 704 for determining what frequencies are available in each cell, an analysis unit 705 for analysing when the number of available frequencies is insufficient, and a generation unit 706 for generating the frequency hopping sequences. The communication unit may effectively act as a central controller and may pass information between the other functional blocks.

An example of the functional blocks that may be comprised in a communication device according to one embodiment of the invention are shown in FIG. 8. The communication device, shown generally at 801, comprises a communication unit 803 connected to an antenna 802 for transmitting and receiving messages. The communication device further comprises a storage unit 804 for storing its frequency hopping sequence and a time sharing unit 805 for effecting time sharing when too few frequencies are available. The communication device further comprises a negotiation unit 806 for negotiating time sharing arrangements with neighbouring communication devices. The communication unit may effectively act as a central controller and may pass information between the other functional blocks.

The apparatus in FIGS. 7 and 8 are shown illustratively as comprising a number of interconnected functional blocks. This is for illustrative purposes and is not intended to define a strict division between different parts of hardware on a chip. In practice, the communication device preferably uses a microprocessor acting under software control for implementing the methods described herein. In some embodiments, the algorithms may be performed wholly or partly in hardware.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. 

1. A communication device that forms part of a network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the communication device being configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in its neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with those communication devices on a time division basis.
 2. A communication device as claimed in claim 1, configured to share the insufficient number of frequencies with its neighbouring communication devices responsive to a communication from a network controller.
 3. A communication device as claimed in claim 2, configured to share the frequencies responsive to a command from the network controller.
 4. A communication device as claimed in claim 2, configured to determine that it should share the frequencies responsive to a frequency hopping sequence received from the network controller, responsive to the number of frequencies comprised in the frequency hopping sequence being below a predetermined threshold.
 5. (canceled)
 6. A communication device as claimed in claim 1, configured to negotiate with its neighbouring communication devices to determine how the insufficient number of frequencies should be shared between them.
 7. A communication device as claimed in claim 1, configured to: communicate with the at least one terminal comprised in its cell via a series of frames, each frame comprising a downlink portion and an uplink portion; and share the insufficient number of frequencies with its neighbouring communication devices by communicating the downlink portion of a frame in one timeslot on a frequency in its frequency hopping sequence and the uplink portion of the same frame in a different timeslot on the same frequency.
 8. A communication device as claimed in claim 67, configured to communicate the downlink portion and the uplink portion of the frame in timeslots arranged such that the time from the start of the timeslot for the downlink portion to the end of the timeslot for the uplink portion is less than or equal to the duration of a frame via which the communication device normally communicates with the at least one terminal.
 9. A communication device as claimed in claim 6, configured to communicate the downlink portion and the uplink portion of the frame in timeslots that are separated by one or more timeslots allocated to its neighbouring communication device(s), such that the at least one terminal perceives the one or more timeslots allocated to the neighbouring communication device(s) as being parts of the frame that are allocated to other terminals in the cell.
 10. (canceled)
 11. A communication network comprising: a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell; each communication device being further configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with those communication devices on a time division basis.
 12. A communication network as claimed in claim 9, comprising, in each of the plurality of cells, a communication device that forms part of a network comprising a plurality of cells, each comprising a communication device configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the communication device being configured to, responsive to a determination that there are an insufficient number of frequencies available to provide frequency hopping sequences for it and the communication devices in its neighbouring cells that are sufficiently distinct from each other, share the insufficient number of frequencies with those communication devices on a time division basis.
 13. A communication network as claimed in claim 9, configured to determine that there are an insufficient number of frequencies available for a group of two or more neighbouring communication devices if the frequency hopping sequences derivable from those available frequencies require two or more neighbouring communication devices to use the same frequency for a length of time that is likely to lead to unacceptable packet loss.
 14. A communication network as claimed in claim 9, comprising a controller configured to determine that there are an insufficient number of frequencies available to provide frequency hopping sequences for a group of two or more neighbouring communication devices.
 15. A communication network as claimed in claim 11, wherein the controller is configured to communicate that there are an insufficient number of frequencies to the two or more communication devices wherein the controller is configured to instruct the two or more communication devices to share the insufficient number of frequencies on a time division basis.
 16. (canceled)
 17. A communication network as claimed in claims 9, configured to allocate neighbouring communication devices the same length of time on each of the insufficient number of frequencies.
 18. A communication network as claimed in claims 9, configured to allocate neighbouring communication devices different lengths of time on each of the insufficient number of frequencies.
 19. A communication network as claimed in claims 9, configured such that each of the communication devices communicates with its at least one terminal via frames having the same duration, those communication devices being configured to, when sharing a frequency with a neighbouring communication device, commence transmitting each frame at a different time from the neighbouring communication device so that the frames of one communication device are offset from those of another.
 20. (canceled)
 21. (canceled)
 22. (canceled)
 23. A controller for determining a frequency hopping sequence for a communication device that is configured to communicate with at least one terminal via a frequency hopping sequence associated with its respective cell, the controller being configured to: determine that there are an insufficient number of frequencies available to provide frequency hopping sequences for two or more communication devices in neighbouring cells that are sufficiently distinct from each other; and instruct those two or more communication devices to share the insufficient number of frequencies between them on a time division basis.
 24. A controller as claimed in claim 17, configured to determine that there are an insufficient number of frequencies available for a group of two or more neighbouring communication devices if the frequency hopping sequences derivable from those available frequencies require two or more neighbouring communication devices to use the same frequency for a length of time that is likely to lead to unacceptable packet loss.
 25. A controller as claimed in claim 17, configured to allocate neighbouring communication devices the same length of time on each of the insufficient number of frequencies.
 26. A controller as claimed in claim 17, configured to allocate neighbouring communication devices different lengths of time on each of the insufficient number of frequencies.
 27. (canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled) 